This invention relates to an electrostrictive effect element, and more particularly to an electrostrictive effect element as actuator in the fields of mechatronics and so on. It also relates to the process of manufacturing such elements.
The electrostrictive effect element is one capable of converting electrical energy into mechanical energy by electrostrictive effect to produce minute, precise mechanical displacement, and generally consists of a material exhibiting electrostrictive effect, such as ceramic, provided with opposite electrodes between which voltage is to be applied.
Preferable electrostrictive effect elements used in the field of mechatronics and so on, are of small-size and capable of producing large displacements at low voltages. So far have been developed the thin film technology for forming a thinner film of electrostrictive material allowing smaller distance between the opposite electrodes required for gaining stronger electric field at the same voltage, and further lamination technique for laminating a plurality of thus-obtained thin electrostrictive effect elements. By application of these, various types of electrostrictive effect elements have been put to practical use.
As an example of such electrostrictive effect element is disclosed in U.S. Pat. No., 4,681,667, a laminated-type electrostrictive effect element which is made by laminating electrostrictive ceramic green sheets and internal electrodes alternately, and then sintering this laminate. All the side faces of every other internal electrode are covered with an insulating material such as glass, and the remaining internal electrodes with side faces exposed are connected to a pair of external electrodes to which a pair of lead wires are soldered. When a voltage is applied to this electrostrictive effect element, adjacent internal electrodes on the opposite sides of each electrostrictive ceramic material work as opposite electrodes to each other, thus displacement being induced in the direction of lamination.
Besides the laminated-type electrostrictive effect element above-mentioned, there is another called stacked-type which is made by sintering discrete electrostrictive ceramic sheets, then laying an internal electrode on both surfaces, respectively, of each sintered sheet, and stacking these into an integrated structure.
These electrostrictive effect elements described above are disadvantageous in high manufacture cost, poor reliability of product, and little promise of large effective displacement. The reasons for these will be set forth under.
Firstly concerning laminated-type electrostrictive effect element: the aforesaid covers of insulating glass material used for insulation are made usually in the process of depositing glass by electrophoretic technique, and then sintered. In association with this, processing steps including formation of tentative electrodes and sintering are needed. Besides such glass deposition cannot carried out at a time on both sides of the laminate, and hence must be done one after the other, for which it takes a longer time. Because of these, it is not easy to increase of product yield.
It is effective for increasing the displacement of electrostrictive effect element to decrease the distance between internal electrodes. Considering the laminated-type electrostrictive effect element, the distance is dependent on the width of the insulating glass cover (referred to as glass insulation hereinafter). By the technique at present, the possible smallest thickness of the electrostrictive ceramic sheet is about 10 .mu.m while the uniform glass insulation layer can be formed at thicknesses about 40 .mu.m or more which corresponds by calculation to the thickness of electrostrictive effect material of about 70 .mu.m. It therefore is impossible to profitably apply the thin film technique of ceramic to above-mentioned structure.
In addition, the insulation capability between external and internal electrodes deteriorates due to movement within the glass insulation layer, of ionized metal derived from external and internal electrodes, and the occurrence and extension of microcracks associated with the displacement induced when in use, and hence the reliability is insufficient.
Concerning the stacked-type electrostrictive effect element, it is difficult to obtain thin uniform electrostrictive ceramic sheets because of occurrence of curve or undulation in the process of sintering discrete electrostrictive ceramic material. Accordingly the minimum value of distance between internal electrodes that can be set is 200 .mu.m. As compared with the laminated-type electrostrictive effect element, it can responds only with small displacement to impression of the same voltage, i.e. the same displacement is induced only by application of high voltage, which reflects an expensive power supply.